This invention is directed to a spatial light modulating semiconductor device, which is line-addressed and of compact size. More particularly, the invention concerns a line-addressable spatial light modulator, wherein a deflectable metallized membrane defining a deformable mirror is mounted above a semiconductor substrate in spaced relationship with respect thereto. The membrane cooperates with a matrix array of floating conductive field plates disposed on an insulating layer covering the substrate to define an array of air gap capacitor cells addressable by an x-y matrix array of field effect address transistors formed in the substrate with the degree of membrane deflection of each air gap capacitor cell being dependent upon the signal on the corresponding field effect address transistor.
The development of a solid state spatial light modulator has been the subject of numerous research efforts in recent years. One application for such a solid state spatial light modulator is in combination with a large matrix-area charge coupled device light imager for use in obtaining real-time, two-dimensional optical transforms. As one example of past efforts in the development of a satisfactory solid state spatial light modulator, it has been proposed to locate a membrane modulator on the backside of a large matrix area charge coupled device (CCD) light imager. In this type of light modulator, electronic charge is input into the front side of the CCD imager, with the entire frame being subsequently transferred to the backside membrane modulator structure through the semiconductor substrate of thinned silicon material. This approach is subject to severe manufacturing difficulties due to the inherently low yield of manufactured low-defect, large-area metal-oxide-semiconductor (MOS) CCD devices. Further complications may arise in that device thinning and subsequent additional backside processing are required to provide the support for the membrane modulator, while flatness and the frontside CCD operation are maintained. A final complication occurs in attempting the apply the membrane to the silicon substrate and the subsequent electrical bonding to both the CCD and the membrane. This type of solid state light modulator involves device processing and patterning on both sides of the semiconductor substrate.
Heretofore, line-addressable spatial light modulators wherein an array of field effect (MOS) transistors are employed for addressing purposes have exhibited a considerable amount of fixed pattern noise which is introduced by the different threshold shifts in the analog amplifiers at the serial-to-parallel converter output.
Spatial light modulators of the membrane type are described in "The Membrane Light Modulator and Its Application in Optical Computer"-Preston, Optica Acta, Vol. 16, No. 5, pp. 579-585 (1969) and in "A Membrane Page Composer"-Cosentino et al, RCA Review, Vol. 34, pp. 45-79 (March 1973). Heretofore, membrane-type light modulators have been subject to the leakage of residual light through the metallized membrane layer to the surface of the silicon substrate, thereby producing a photo-induced charge in the depletion region of the air gap capacitor associated with the portion of the metallized membrane through which residual light is transmitted. Efforts to avoid this problem have included an increase in the thickness of the metal layer of the membrane to render the membrane more opaque to incident light. However, the thickened metal layer of the membrane structure reduces the deflection response of the membrane, thereby increasing the required operation voltages to an undesirable degree for effecting a desired membrane deflection response. Another problem with previous spatial light modulators employing deformable membrane structures as a component thereof occurs when the collapse voltage of the membrane is exceeded. In this instance, the membrane collapses into engagement with the gate oxide surface of the semiconductor substrate when the voltage applied to the membrane exceeds a predetermined magnitude, and the membrane remains bonded to the gate oxide surface even after removal of the voltage applied to the membrane.